Deglaciation and relative sea-level chronology, Casco Bay Lowland and lower Androscoggin River valley, Maine

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Special Paper of the Geological Society of America

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Book Chapter - Open Access

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Retreat of the late Wisconsinan Casco Bay sublobe of the Laurentide ice sheet and relative sea-level history in the Casco Bay Lowland and lower Androscoggin River valley, Maine, is represented by deglacial-phase deposits, including glaciomarine deltas, fans, stratified end moraines, and fossiliferous glaciomarine mud, as well as nearshore deposits associated with synglacial sea level and postglacial uplift. Deglaciation was systematic, but interrupted by minor readvances in the region along an eastnortheast-trending lobate ice margin, oblique to the present coastline, and is documented by a consistent alignment of ice-marginal deposits.

Radiocarbon ages on marine shells (uncorrected for the marine-reservoir effect) and on terrestrial organics imply that southwestern coastal Maine was deglaciated by 15 ka, the Casco Bay region by 14 ka, and the White Mountain foothills by 13 ka. At a site along the present Maine coast in Phippsburg, isostatic adjustment had begun by 13.6 ka, and by this time relative sea level had already fallen from a maximum local marine limit of 64 m above sea level (asl) to 49 m asl. A relative sea-level curve for the Casco Bay Lowland and lower Androscoggin River valley is similar in form to the regional sea-level curve for Maine, but differs because it indicates that deglaciation and emergence of southwestern Maine occurred earlier than previously reported.

Radiocarbon ages on marine shells are the most common data used to infer age of deglaciation in coastal Maine. The deglaciation of the Casco Bay Lowland occurred between 14 and 13 ka, during the time of Heinrich event 1 in the North Atlantic Ocean. By 12.8 ka, ice was north of the study area and isostatic emergence had caused relative sea level in the lower Androscoggin Valley to drop to 45 m asl. The Brunswick sand plain, a coastal braid-plain delta, had begun forming by the time the marine regression dropped to 35 m asl. On the basis of the local sea-level curve, the sand plain formed 12.5 to 12 ka, within the time span of meltwater pulse 1-A, when falling relative sea level may have stabilized as a response to the meltwater-induced eustatic sea-level rise.


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